This invention relates in general to electrostatography and, more specifically, to a flexible electrophotoconductive imaging member having an electrically conductive ground strip layer.
In the art of xerography, a xerographic plate comprising a photoconductive insulating layer over an electrically conductive layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Other electrostatographic imaging devices utilizing an imaging layer overlying a conductive layer include electrographic devices. For flexible electrographic imaging members, the conductive layer is normally sandwiched between a dielectric imaging layer and a supporting flexible substrate. Thus, generally, flexible electrophotographic imaging members generally comprise a flexible recording substrate, a thin electrically conductive layer, and at least one photoconductive layer and electrographic imaging members comprise a conductive layer sandwiched between a dielectric imaging layer and a supporting flexible substrate. Both of these imaging members are species of electrostatographic imaging members.
In order to properly image an electrostatographic imaging member, the conductive layer must be brought into electrical contact with a source of fixed potential elsewhere in the imaging device. This electrical contact must be effective over many thousands of imaging cycles in automatic imaging devices. Since the conductive layer is frequently a thin vapor deposited metal, long life cannot be achieved with an ordinary electrical contact that rubs directly against the thin conductive layer. One approach to minimize the wear of the thin conductive layers is to use a grounding brush such as that described in U.S. Pat. No. 4,402,593. However, such an arrangement is generally not suitable for extended runs in copiers, duplicators and printers.
Still another approach to improving electrical contact between the thin conductive layer of flexible electrostatographic imaging members and a grounding means is the use of a relatively thick electrically conductive grounding strip layer in contact with the conductive layer and adjacent to one edge of the photoconductive or dielectric imaging layer. Generally the grounding strip layer comprises opaque conductive particles dispersed in a film forming binder. This approach to grounding the thin conductive layer increases the overall life of the imaging layer because it is more durable than the thin conductive layer. However, such relatively thick ground strip layer are still subject to erosion and contribute to the formation of undesirable "dirt" in high volume imaging devices. Erosion is particularly severe in electrographic imaging systems utilizing metallic grounding brushes or sliding metal contacts.
Also, in systems utilizing a timing light in combination with a timing aperture in the ground strip layer for controlling various functions of imaging devices, the erosion of the ground strip layer by devices such as stainless steel grounding brushes and sliding metal contacts is frequently so severe that the ground strip layer is worn away and becomes transparent thereby allowing light to pass through the ground strip layer and create false timing signals which in turn can cause the imaging device to prematurely shut down. Moreover, the opaque conductive particles formed during erosion of the grounding strip layer tends to drift and settle on other components of the machine such as the lens system, corotron, other electrical components and the like to adversely affect machine performance. For example, at a relatively humidity of 85 percent, the ground strip layer life can be as low as 100,000 to 150,000 cycles in high quality electrophotographic imaging members. Also, due to the rapid erosion of the ground strip layer, the electrical conductivity of the ground strip layer can decline to unacceptable levels during extended cycling.
Thus, the characteristics of flexible electrostatographic imaging members utilizing ground strip layer exhibit deficiencies which are undesirable in automatic, cyclic electrostatographic imaging systems.